1. Field of the Invention
The invention relates to a process for manufacturing a sheet composed of a plurality of ceramic filaments with a metal matrix and to the bonding together of the filaments.
2. Discussion of the Background
In the field of aeronautics in particular, one constant objective is to optimize the strength of components for a minimum mass and minimum size. Thus, certain components may from now on include an insert made of a metal matrix composite, the component possibly also being monolithic. Such a composite comprises a metal alloy matrix, for example a titanium (Ti) alloy, in the core of which fibers extend, for example silicon carbide (SiC) ceramic fibers. Such fibers have a tensile strength very much greater than that of titanium (typically 4000 MPa compared with 1000 MPa). It is therefore the fibers that take the loads, the matrix in the metal alloy providing a function of binder with the rest of the component and also the function of protecting and isolating the fibers, which must not come into contact with one another. Furthermore, the ceramic fibers are resistant to erosion but necessarily have to be reinforced with metal.
These composites may be used in the formation of disks, shafts, ram bodies, casings, and spacers, such as reinforcements for monolithic components such as blades, etc.
To produce such a composite insert, filaments called “coated filaments”, comprising a metal-coated ceramic fiber, are manufactured beforehand. The metal gives the filament the elasticity and the flexibility needed for handling it. Preferably, a very fine carbon or tungsten filament lies at the center of the fiber, along its axis, this carbon filament being coated with silicon carbide, while a thin film of carbon is provided at the interface between the fiber and the metal, in order to provide a diffusion-barrier/buffer function during differential thermal relaxation that occurs as the liquid metal deposited on the fiber cools.
The composite filaments, or coated filaments, may be manufactured in various ways, for example by vapor deposition of metal in an electric field, by electrophoresis using a metal powder, or else by dip-coating ceramic fibers in a bath of liquid metal. A coating process, in which ceramic fibers are dipped into a liquid metal, is presented in patent EP 0 931 846 in the name of the Applicant. Manufacture by this process is rapid.
In the known processes for producing a component with an insert made of a metal alloy matrix composite, the coated filament is formed from a workpiece called a preform. The preform is obtained by winding the coated filament between two metal retaining flanges that extend around a central mandrel. The winding is in a spiral, the preform obtained being in the form of a disk, the thickness of which is that of the coated filament. To ensure cohesion of the preform, the retaining flanges include apertures through which a material providing a bonding function, for example an acrylic resin, is sprayed.
FIG. 1 shows schematically one embodiment of an operation for the manufacture of a component with a composite insert. A plurality of preforms 1, each in the form of a disk, are stacked in a container 2 of cylindrical overall shape. The container has an annular cavity 3, the sectional shape of which, transverse to the axis 4 of the container, is that of the preforms 1. The preforms 1 are stacked until the entire height of the cavity 3 is filled. Typically, 80 preforms are thus stacked. This operation is manual.
It is then necessary to perform a binder-removal operation followed by degassing, so as to eliminate the binder, for example an acrylic resin, from the preforms 1. This is because no contaminating element must remain, when cold and hot, in contact with the titanium during the pressing stage.
An annular lid 5, having a projection 6 of shape complementary to that of the annular cavity, but of smaller axial dimension, is placed on top of the container 2, the projection 6 being brought into contact with the upper preform 1. The lid 5 is welded to the container 2, for example by electron beam welding, the cavity preferably being maintained in a vacuum. The assembly then undergoes hot isostatic pressing. During this operation, the insert composed of juxtaposed coated filaments is compacted and the metal sheaths of the coated filaments being welded together and welded to the walls of the cavity 3 of the container 2 by diffusion, in order to form a dense assembly composed of the metal alloy (for example a titanium alloy) within which the ceramic fibers (for example SiC fibers) extend annularly.
A cylindrical component is obtained that includes an insert made of a composite, resulting from the compaction of the stacked preforms 1. This component may optionally undergo a stress relaxation treatment, making it possible to compensate for the differential expansion between the ceramic fibers and the metal, in which they are embedded, when the assembly cools.
The component is then generally machined. For example, if the objective is to manufacture a one-piece compressor disk—the term “one-piece” meaning that the blades are formed from a single component with the disk—the container, including its composite insert, is machined so as to form a one-piece bladed disk or “blisk”, one part of the rim supporting the blades including the composite insert. The rim is of much smaller dimensions than the rims of conventional metal disks, thanks to the high stiffness and high strength values conferred on the assembly by the ceramic fibers of the ceramic composite, contained in the mass of the rim. In particular, such a rim may be in the form of a simple ring.
This process for manufacturing a component with a composite insert has drawbacks, and is difficult to exploit on an industrial scale owing to the length, complexity and precision required of its steps.
Firstly, since the ceramic fibers are brittle, the operations on the coated filaments must above all prevent any contact between them, and the welding of coated filaments has not been envisaged hitherto.
Furthermore, the binder-removal and degassing operations are lengthy, and there is never certainty that all of the binder has been removed. To ensure complete disappearance of the binder, necessary in particular for the correct subsequent behavior of the titanium alloy, several binder-removal and degassing steps are needed. This lengthens the total duration of the process and increases its overall cost.
In addition, should the filament break while it is being wound between the two flanges, it is necessary to form a new preform in so far as at the present time no means exist for solving the problem and resuming the winding.
Moreover, the step of positioning the coated filament preforms in the container is currently manual. The cost of the operation and in particular its precision are affected thereby. Now, the positioning of the coated filament in the container is a critical factor in the manufacturing sequence in so far as it determines the performance of the composite, with a very great influence of the orientation of the ceramic fiber according to the principal stresses of the component. It also determines the quality of the composite, by preserving the integrity of the ceramic fiber, during the various steps in the manufacture of the component. Lastly, it determines the final cost of the component, again because the operations of positioning the coated filaments are relatively lengthy and carried out manually. The positioning of the filaments in the container should therefore benefit from being improved.
Also known is a process that comprises the formation of a sheet of a plurality of metal filaments side by side, in which the filaments, placed parallel to one another and in mutual contact, are made to pass between two rolls forming a rolling mill. This forces the filaments to be bonded together. Such a process cannot easily be applied to coated filaments such as those used in the formation of a component with an insert made of a composite in accordance with the invention, since these coated filaments include a very brittle ceramic fiber at their center, which runs the risk of being broken by the lack of precision of such a process. Such a break would nullify all the advantages associated with the presence of ceramic fibers within the composite insert. In addition, this process, necessarily carried out hot, results in contamination of the surface of the titanium sheath, which necessarily then has to be removed.